1. Field of the Invention
This invention relates to a method for modifying the curvature of the cornea with particular application in the correction of myopia.
2. Background of the Invention
Over the last few years myopic keratomileusis has been subjected to rapid and significant technical and surgical evolution to such a degree that it is now the elective technique for the correction of moderate-severe ametropias and the main subject of debate in international congresses which deal with keratorefractive surgery. It appears that with slight ametropias, the reliability and the stability of the results offered by the technique of Photo Refractive Keratectomy (PRK (the modification of the corneal surface using excimer lasers)) is reasonably satisfactory. However, it also appears that current PRK techniques are not in a position to provide adequate predictability and long-term stability for the correction of more severe refractive defects. These objectives are reached more satisfactorily if the refractive procedure is performed on the intrastromal site; this will also avoid direct damage to the Bowman's membrane, a complication which is responsible for the well-known post-ablative haze and the expected long-term regression of the results.
The refractive procedure, when performed directly on the stroma will save the Bowman's membrane, reduce tissue reactivity, lower the dependency on corticosteroid treatment and more importantly, obtain a refractive result that is both predictable and stable over time. For these reasons, the international scientific interest is largely directed towards intrastromal keratomileusis particularly following the development of excimer lasers and the techniques for photoablation of the stroma. The most recent approach towards keratomileusis is the so-called LASIK technique.
The LASIK technique involves the use of a keratotome, an electromechanical and pneumatic device which performs circular corneal resections to create flaps with more or less parallel faces and pre-determined diameter and thickness. The keratotome or microkeratome, depending on the specific model, may also be automated. The instrument is also fitted with a stop mechanism to block the run of the blade (or other cutting device) and prevent a full circle (360.degree.) corneal disk from being cut. This allows the surgeon to perform a cut which stops 1-1.5 mm from the end of the run and thus creates a hinge. This technique allows the surgeon to lift the flap which has been cut, place it on the nasal side, and expose the underlying stromal bed to the refractive in situ procedure using the excimer laser. The flap is then replaced and remains in position also under the influence of the uncut portion (the hinge). This technique allows the surgeon to perform a lamellar cut on the front surface of the cornea involving the superficial layer of the cornea. This cut extends in a circular direction but stops before the completion of 360.degree. so that the superficial layer (flap) remains attached to the remaining part of the cornea in the nasal zone by means of this hinge.
The superficial layer (flap) is then raised and moved to one side to expose the surfaces underlying the cornea; these are subjected to in situ ablation using the excimer laser and the curvature of the cornea is modified to the desired value. Finally the superficial layer (flap) which was previously moved to the nasal side is returned to the surface, which have now been ablated, and it (the flap) adheres to the stromal bed without sutures being used.
There have been some problems with this technique which have come to light during the course of the trials. The superficial layer which remains attached to the underlying cornea in the nasal sector during the operation (the hinge which remains attached and has not been cut) cannot prevent the movement of the superficial layer during the vertical blinking movements of the eyelid. The superficial layer remains attached to the underlying tissue more for the effect of the endothelial pump rather than for purely mechanical reasons. So the superficial layer with the nasal hinge can move and the movement is one of the most frequent complications observed in the post-operative period during the learning curve of this procedure. The complication will arise particularly if the flap is re-positioned incorrectly at the end of the operation, if the superficial layer does not adhere sufficiently to the underlying stromal bed in the immediate post-operative and more generally if there are defects in the epithelium or excessive lachrymation.
Moreover with this technique, it can happen that the superficial layer is positioned incorrectly. It can be replaced in a position which is slightly decentered upwards, downwards or nasally. It may also happen that it has not been smoothed sufficiently which will result in the formation of intralamellar folds which will persist post-operatively and interfere with the visual function. In the best scenario, the blinking action of the eyelids will not interfere with the flap but in the worst scenario it can actually increase the degree of dislocation and/or the number of folds.
Another problem inherent to this operating technique is the fact that the superficial layer which has been cut, has been placed nasally for the time necessary to perform the refractive procedure with the laser. In the event of small-diameter lamellar cuts or ablations on large optical zones there is the risk that the stromal surface of the flap will also accidentally be ablated (the part that has been exposed by raising the flap and turning it over). In order to avoid this possibility, the hinge should be protected.
In actual practice, the nasal hinge limits the extension of the ablation in the nasal sector which may reflect on the final refractive outcome and more specifically the visual function (the area not treated nasally). This is particularly important in the case of with-the-rule astigmatism because the ablation is done along the horizontal axis and the treatment involves wide-ablation zones. Laser ablation involves the removal of tissue. The central depth of the ablation is proportional to the degree of myopia, the optic zone chosen and whether or not multi-zone treatments are being used. If, for example, the surgeon performs a treatment for 10 myopic diopters on a single 5.5 mm zone, the depth of the ablation at the center will be about 100 microns. So therefore, between the center of the ablation and the zone which has not been treated, there will be a difference in depth of about 100 microns.
The intrastromal `vacuum` left by the laser ablation is filled by the lamella or the superficial layer which adapts to the underlying bed. However, there are a number of problems. The formation of lamellar microfolds which are more numerous and obvious when the ablation is deeper and there is no transition zone, can, post-operatively, negatively affect the visual performance. In the classical operating technique LASIK, under the influence of the nasal hinge, the lamella can not be positioned uniformly around the 360.degree.. This involves nasally--oriented micromovements of the lamella and influences the arrangement of the folds which will extend nasally.